I'd like to nominate The Fairy Godmother by Mercedes Lackey. It's the first book in Five Hundred Kingdoms series.

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Elena Klovis was supposed to be her kingdom's Cinderella -- the mistreated stepsister who fatefully falls in love with a prince, marries him, and lives happily ever after -- except that the prince is only a child! When Elena's stepmother and her two wicked stepsisters hastily leave town to escape debtors, she is left alone in the empty house to fend for herself. A 21-year old woman with no education, no money, and no hope of ever marrying a highborn man, Elena has only one option: to try to be hired as a servant. (At least she'll be paid for her hard work!) Then fate intervenes. She is taken on as an apprentice to a fairy godmother and is introduced to a life of magic and supernatural creatures.

Elena eventually becomes a fairy godmother herself and revels in helping steer people in the right direction. But when she helps three princes (one extremely handsome, by the way!) on a quest, will she let her emotions get in the way of her responsibilities?

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The prolific Lackey draws on the darker, Brothers Grimm side of fairy lore for her enchanting tale. In the land of the Five Hundred Kingdoms, the Tradition, that ineffable magic, holds the promise of happily-ever-after for all deserving young maidens and courteous princes charming. But the Tradition also leads some in its thrall to pain, suffering and gruesome death. Feisty 19-year-old Elena Klovis seems destined to be an Ella of the Cinders (Cinderella), at the mercy of her wicked stepmother and greedy stepsisters. To escape their clutches, Elena tries to get work as a maidservant, but her fairy godmother, Madame Bella, has other plans for her. Elena becomes Madame Bella's apprentice, doing her best, among other challenges, to ensure that evil does not subvert Tradition. The only problem is that fairy godmothers are not themselves allowed to fall in love. It's up to Elena, who has vowed to reform a wayward prince, to tease out the threads of a new Tradition. Lackey has created an intelligent, self-possessed heroine with whom many readers will identify.